In my work I model the observed broad-band synchrotron and IC emission from relativistic flows -- mainly in active galaxies, but recently extended to galactic microquasars and Gamma Ray Bursts (GRBs) -- to understand the dynamics of these flows. This is important not only for understanding the physics of these powerful collimated outflows themselves; it also helps us put these flows in a bigger picture (e.g. the feedback of the jets of an active galaxy can heat up can heat up the host galaxy custer gas).
Relativistic flows decelerate and it shows!
Recently, I have argued, together with Demos Kazanas and Eric Perlman,
using current multiwavelength data (from radio up to TeV energies) that
the jets of active galaxies decelerate
at sub-pc scales , continue decelerating mildly at
kpc scales and, eventually decelerate to
non-relativistic velocities
at the terminal hot spots of powerful radio
galaxies and quasars. In the process, Kazanas and I discovered a novel
emission process taking place in relativistic decelerating flows:
Upstream Compton (UC) scattering , a process in which synchrotron
photons from its slower
downstream section serve as seed photons for Compton scattering by
electrons in its upstream faster part.
The resulting UC emission is stronger and more beamed than anticipated
from constant Lorentz factor flows.
A probe of the matter content of powerful jets
One of the most embarrassing problems we face is that we still do not
know if the
jets ar made out or normal electro-proton plasma,
or if they are made out of electron-positron pair plasma. Obviously, any
intermediate solution is open. Because we only "see" the relativistic leptons
that radiate, it is very difficult to constraint the "silent" population
of protons and/or cold leptons. Recently, we
(Georganopoulos et al 2005)
noticed that there are some relativistic quasar jets that are dark
between the core and the first knot at distances of the order of ~100 Kpc from
the core. We interpreted the multiwavelength darkness as an indication that
the leptons carried by the flow are practically cold at the flow comoving
frame. These cold leptons, moving balistically with the bulk Lorentz factor
of the jet, will upscatter the cosmic microwave background (CMB) to near-IR
energies. One can then calculate the flux level of this "bulk Compton"
emission as a function of jet matter content, assuming minimum power
conditions for the flow. Our method was applied by
Uchiyama et al (2005)
in PKS 0637-752 using SPITZER, concluding that a pair jet is excluded.
We were recently
awarded
HST NICMOS time for the same source that will permit
us to set strong constraints on the lepton to proton ratio for the jet plasma.
If you want to know more about my research, please take a look at my work in ADS and astro-ph.